PR Title: feat(mvu): productize the phase 6 workflow

[dancancer]

Summary

• add a /session MVU debugger with strategy selection, route tracing, status bar preview, template preview, and node snapshot/schema/delta inspection
• wire the selected MVU strategy through the dialogue runtime, keep text-delta as the default path, and add explicit function-calling / extra-model extension behavior
• harden MVU protocol hygiene so UpdateVariable blocks stay available for internal persistence but do not leak into visible output, regex processing, or vector memory
• add minimal extra-model runtime persistence, node-level MVU trace storage, committed phase 6 fixtures, regression tests, and phase 6 workflow/capability docs

Test Plan

• pnpm verify:stage

Notes

• verified locally: lint, typecheck, vitest, and production build all pass
• branch: codex/phase-6-mvu-productization
• commit: b7e74b0